New alloy systems like Co-Re, which contain a high amount of refractory elements, are promising candidates for high-temperature applications. The development of new high-temperature materials aims at a favourable combination of following properties: (i) phase stability, (ii) mechanical properties and (iii) corrosion resistance. Although the reference alloy Co-17Re-23Cr shows satisfactory high-temperature strength, it possesses poor oxidation resistance. The high Cr content in this alloy leads to the formation of coarse particles of the σ phase (Cr2Re3) which deteriorates the mechanical properties. It was found in an earlier study that improved oxidation behaviour can be realized by adding 15 at.% Ni to the reference alloy. In addition, Ni decreases the volume fraction of the σ phase. In order to understand the basic mechanisms of this Ni effect, the study presented focused on the microstructural evolution of two Co-Re-Cr-Ni alloys with different Ni contents (15 and 25 at.%). The correlation of the microstructure with the oxidation resistance and the alloy hardness was investigated. The microstructural examinations were performed using scanning electron microscopy, X-ray diffraction and differential scanning calorimetry. Oxidation tests were conducted by continuous thermogravimetry in the temperature range 800-1100°C. For the quick screening of the mechanical properties, hardness tests were carried out. It was found that the Ni-containing Co-Re-Cr alloys exhibit very fine lamellar σ particles. Furthermore, this morphology of the σ phase remains stable during prolonged high-temperature exposure. With increasing Ni concentration and temperature (from 800 to 1000°C) the face-centred cubic γ-phase becomes stable strongly affecting the alloy properties. The experimental results showed that the enhanced volume fraction of the fcc phase leads to decreased alloy hardness. It was proven that the formation of the σ phase and the ε↔γ phase transformation are correlated processes which are accelerated in the alloy with 25 at.% Ni. It seems that this phenomenon can be attributed to an enhanced Cr diffusion in Ni-containing alloys. This hypothesis appears to be also reasonable in terms of the improved oxidation resistance of Co-Re-Cr-Ni alloys as the Ni additions facilitate the fast formation of a continuous chromia layer. In our future studies, the diffusion coefficients of Cr as a function of the Ni content will be determined to verify this idea.